1. Field of the Invention
This invention relates to hypodermic syringes.
2. Description of the Related Art
Plastic hypodermic syringes are mass-producible, and are intended to be sterilized or discarded after only one use to prevent the spread of blood-borne diseases such as hepatitis and AIDS from one user to the next. For example, if an individual using a syringe is carrying a blood-borne disease, a subsequent user of the syringe or a person inadvertently stuck with the needle runs a great risk of contracting the disease.
Plastic hypodermic syringes which are not pre-filled are usually loaded by the action of a reciprocable plunger which creates a reduced pressure in the barrel and results in the intake of fluid (medication) into the barrel. Such syringes depend on a rubber seal attached to the plunger to insure that a reduced pressure can be effected because of the wider tolerances experienced in plastic molding, in contrast to precisely ground glass syringes which do not require a seal. Withdrawal of the plunger with seal attached is now messy since the seal has been covered with fluid. Also, the syringe requires a separate needle cover to protect individuals from accidental and potentially hazardous puncture wounds and inadvertent disease transmission. Separate needle covers drive up the cost of the unit.
There is another type of conventional syringe that can be used with pre-filled ampoules. The syringe is provided with a needle having points at both ends, where one end of the needle extends into the barrel of the syringe. When an ampoule is inserted into the barrel, the internal point of the needle punctures the seal of the ampoule, and medicine is placed in fluid communication with the needle. Thereafter, a plunger is placed in the open end of the barrel; when the plunger is depressed, it forces down the stopper of the ampoule, which forces medicine out of the ampoule and through the needle.
There is a drawback to this type of syringe as well. Typically, the needle extends into the barrel (and thus into an inserted ampoule) so much that a significant distance exists between the fluid entry point of the opening in the needle and the bottom of the ampoule. As a result, any medicine in the ampoule below the point of the opening in the needle will not be forced out of the ampoule when the plunger is fully depressed. This "dead space" created at the bottom of the ampoule wastes medicine.
Moreover, typical syringes possess a needle mounted in the bottom wall of a barrel. The amount the needle extends from the barrel is fixed, and may not be altered. This reduces the flexibility of the use of the syringe, as some injections are supposed to be made deeper into the tissue of the patient than others (i.e., intramuscular injections are usually deeper than intravenous injections). Accidents may occur when performing an injection that is meant to be close to the surface of the skin, such as inadvertent extra penetration and related tissue damage.